The Notch locus of D. melanogaster is required early in embryogenesis for proper development of the ectoderm. In Notch-deficient flies, cells that should form hypoderm (i.e. skin) form nervous tissue in its place. From DNA sequence studies, Notch produces a protein that is homologous to mammalian clotting factors and growth factors, all of which bind to other proteins in performing their in vivo functions. The structure of the Notch protein, as deduced from DNA sequencing, suggests that it is a call surface-bound ligand. In this proposal we will extend an analysis of Notch and its product as follows: 1) Antibodies to the Notch protein will be used to determine where and when the protein is produced in developing embryos, whether it is membrane-bound, and whether or not the protein is processed to form diffusible, biologically active molecules. 2) Certain mutants at the Notch locus appear to code for hyperactive proteins. The DNA changes associated with these mutations will be determined and related to the structure of the mutant proteins. Antibodies will be used to determine the cellular fate of the mutant proteins in an attempt to understand their hyperactivity. 3) The effects of neurogenic mutations at unlinked loci on Notch expression will be analyzed. 4) Transformation will be used to determine what parts of the Notch protein are essential for its function. A subdivision of function is indicated by genetic analysis of Notch, so correlated maps of specific Notch functions and various segments of this very large protein will be constructed. 5) Since the structure of the Notch protein suggests that it is a cell surface-bound signal or receptor, a Drosophila factor(s) which binds to Notch protein will be sought.